Structural characteristics of grain boundaries in superconducting Bi2Sr2Ca1Cu2Ox fibers

Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) have been used to study grain morphology and grain boundaries in superconducting Bi₂Sr₂CaCu₂ O_x fibers produced by gas-jet fiberization. The narrowest fibers are composed of chains of platelets of about 52×3×0.2 μm³ oriented such that the long direction in each platelet is primarily parallel to the fiber axis. Each platelet has one crystallographic orientation. with the c-axis corresponding to the thin direction. These crystallites commonly join via large-angle (001) twist boundaries. where the ab-planes are parallel but rotated about the c-axis with respect to the next crystallite. More complicated boundaries are also observed. It is noted that it is not clear if having large-angle twist boundaries is a disadvantage in terms of achieving high critical current densities in these fibers. Twist boundaries may encourage a geometry where many crystallites can be stacked one atop the other, maximizing the overlap of the basal planes, which would allow the fiber to accommodate more current in the superconducting state